Re: [PATCH v7 0/3] spapr: nvdimm: Introduce spapr-nvdimm device

On 2/4/22 09:15, Shivaprasad G Bhat wrote:
> If the device backend is not persistent memory for the nvdimm, there
> is need for explicit IO flushes to ensure persistence.
> 
> On SPAPR, the issue is addressed by adding a new hcall to request for
> an explicit flush from the guest when the backend is not pmem.
> So, the approach here is to convey when the hcall flush is required
> in a device tree property. The guest once it knows the device needs
> explicit flushes, makes the hcall as and when required.
> 
> It was suggested to create a new device type to address the
> explicit flush for such backends on PPC instead of extending the
> generic nvdimm device with new property. So, the patch introduces
> the spapr-nvdimm device. The new device inherits the nvdimm device
> with the new bahviour such that if the backend has pmem=no, the
> device tree property is set by default.
> 
> The below demonstration shows the map_sync behavior for non-pmem
> backends.
> (https://github.com/avocado-framework-tests/avocado-misc-tests/blob/master/memory/ndctl.py.data/map_sync.c)
> 
> The pmem0 is from spapr-nvdimm with with backend pmem=on, and pmem1 is
> from spapr-nvdimm with pmem=off, mounted as
> /dev/pmem0 on /mnt1 type xfs (rw,relatime,attr2,dax=always,inode64,logbufs=8,logbsize2k,noquota)
> /dev/pmem1 on /mnt2 type xfs (rw,relatime,attr2,dax=always,inode64,logbufs=8,logbsize2k,noquota)
> 
> [root@atest-guest ~]# ./mapsync /mnt1/newfile ----> When pmem=on
> [root@atest-guest ~]# ./mapsync /mnt2/newfile ----> when pmem=off
> Failed to mmap  with Operation not supported
> 
> First patch adds the realize/unrealize call backs to the generic device
> for the new device's vmstate registration. The second patch implements
> the hcall, adds the necessary vmstate properties to spapr machine structure
> for carrying the hcall status during save-restore. The nature of the hcall
> being asynchronus, the patch uses aio utilities to offload the flush. The
> third patch introduces the spapr-nvdimm device, adds the device tree
> property for the guest when spapr-nvdimm is used with pmem=no on the
> backend. Also adds new property pmem-override(?, suggest if you have better
> name) to the spapr-nvdimm which hints at forcing the hcall based flushes even
> on pmem backed devices.
> 
> The kernel changes to exploit this hcall is at
> https://github.com/linuxppc/linux/commit/75b7c05ebf9026.patch



Applied for ppc-7.0

Thanks,

C.